In tubes used for automobile lines, the main material thereof is undergoing a change from metal to lightweight resin having superior rust resistance in response to demands for elimination of problems with rust generation that have long been caused by the use of deicing materials on roads, prevention of global warming and conservation of energy. Normally, examples of resins used for the tubes of automobile lines include polyamide-based resins, saturated polyester-based resins, polyolefin-based resins and thermoplastic polyurethane-based resins, in the case of single-layer tubes using these resins, the application range thereof had been limited due to inadequacies with respect to heat resistance, chemical resistance and the like.
In addition, in the tubes used for automobile lines, oxygen-containing gasoline blended with low boiling point alcohols such as methanol or ethanol or ethers such as ethyl-t-butyl ether (ETBE) is transported through these tubes from the viewpoint of reducing gasoline consumption and enhancing performance. Moreover, strict exhaust gas regulations have been implemented that include prevention of leakage into the atmosphere attributable to diffusion of volatile hydrocarbons and the like through the walls of these tubes used for automobile lines. In response to these strict regulations, single-layer tubes, consisting only of conventionally used polyamide-based resin, and particularly Polyamide 11 or Polyamide 12 that demonstrate superior strength, toughness, chemical resistance and flexibility, have inadequate barrier properties for the aforementioned chemicals, and require improvement particularly with respect to minimizing permeation of alcohols-containing gasoline.
A method for solving this problem has been proposed that uses a multilayer tube in which is arranged a resin demonstrating favorable resistance to chemical permeation, such as a saponified ethylene/vinyl acetate copolymer (EVOH), polymetaxylylene adipamide (Polyamide MXD6), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-tetrafluoroethylene-hexafluoropropylene copolymer (EFEP), ethylene-chloritrifluoroethylene copolymer (ECTFE), tetrafluoroethylene-hexafluoropropylene copolymer (TFE-HFP, FEP), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer (TFE-HFP-VDF, THV), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride-perfluoro(alkylvinylether) copolymer (TFE-HFP-VDF-PAVE), tetrafluoroethylene-perfluoro(alkylvinylether) copolymer (TFE-PAVE, PFA), tetrafluoroethylene-hexafluoropropylene-perfluoro(alkylvinylether) copolymer (TFE-HFP-PAVE), or chlorotrifluoroethylene-perfluoro(alkylvinylether)-tetrafluoroethylene copolymer (CTFE-PAVE-TFE, CPT) (see, for example, U.S. Pat. No. 5,554,425).
Among these resins, a saponified ethylene-vinyl acetate copolymer (EVOH) has superior resistance to chemical permeation and superior resistance to permeation of gasoline in particular. For example, a fuel line has been proposed that is composed of an outermost layer consisting of Polyamide 12, an adhesive layer consisting of modified polyolefin, an outer layer consisting of Polyamide 6, an intermediate layer consisting of saponified ethylene-vinyl acetate copolymer (EVOH), and an innermost layer consisting of Polyamide 6 (see, for example, Japanese Unexamined Patent Publication No. H3-177683). However, in this fuel line, in the case of using Polyamide 6 for the innermost layer, resistance to sour gasoline formed by oxidation of gasoline (deteriorated fuel resistance) and resistance to calcium oxide (chemical resistance) are inferior. In addition, a multilayer composite has been proposed that is composed of an outermost layer consisting of Polyamide 12, an adhesive layer consisting of at least one type of material selected from the group consisting of Polyamide 6/12 copolymer, Polyamide 12/6 copolymer, Polyamide 612, Polyamide 610 and a mixture of Polyamide 12, Polyamide 6 and a compatibilizing agent, an intermediate layer consisting of saponified ethylene-vinyl acetate copolymer (EVOH), and an innermost layer consisting of Polyamide 6 or Polyamide 12 (see, for example, Japanese Translation of PCT International Application Publication No. 2003-535717 or Japanese Unexamined Patent Publication No. 2003-021276). Similarly, a multilayer composite has been proposed that is composed of an outermost layer consisting of Polyamide 12, an adhesive layer consisting of a mixture of Polyamide 6, Polyamide 12 and polyamine-polyamide copolymer, an intermediate layer consisting of saponified ethylene-vinyl acetate copolymer (EVOH), and an innermost layer consisting of Polyamide 6 or Polyamide 12 (see, for example, Japanese Unexamined Patent Publication No. 2002-210904). This technology proposes that having a polyamide copolymer having a specific composition ratio or a mixture consisting of Polyamide 6, Polyamide 12 and a compatibilizing agent as an adhesive layer interposed between both Polyamide 12 and saponified ethylene-vinyl acetate copolymer.
Moreover, a multilayer tube has been proposed in which is arranged a fluorine-based resin in the inner layer, a saponified ethylene-vinyl acetate copolymer on the outside thereof, and polyterephthalamide (PPA) in the outer layer (see, for example, U.S. Patent No. 2010-0035116).